1. Field of the Invention
The present invention relates, in general, to fluid storage tank monitoring and, more particularly, to a float excitation system which efficiently minimizes the effects of fluid surface tension, allowing float to settle into the fluid to its true buoyancy for high precision leak detection.
2. Statement of the Problem
Leaking fluid product storage tanks represent a significant economic and environmental concern. Product leaks waste valuable product fluid stored in the tanks. Moreover, such leaks can cause water and possibly soil to contaminate the product fluid stored in the tank. More importantly, leaking fluid product storage tanks result in contamination of the surrounding soil and groundwater, which is especially critical when the fluid product is fuel, chemicals, or the like.
Leaks are often so small that volume lost over time is a fraction of the storage capacity of the tank. Over time, however, significant quantities of product fluid are lost. The slow leak rate makes rapid leak detection particularly difficult. The measurement problem is complicated because underground storage tanks are particularly harsh environments in which to make accurate measurements. For example, temperature changes or changes in barometric pressure affect the product fluid level and the measurement apparatus to such an extent that the volume change caused by a slow leaks is difficult.
Leak detectors, also called fluid level detectors, usually include a float that rides on the surface of the product fluid. The float is attached to some form of position transducer that generates a signal indicating the position of the float relative to some fixed reference position. The float position is monitored over a long period of time to detect leaks. Often, the tank is in use (i.e., having product added and removed) during the measurement, so the leak detector must distinguish between the slow, steady volume change caused by a leak and the more dramatic change caused by normal use.
U.S. Pat. No. 4,850,223 entitled "LEAK DETECTOR", issued on Jul. 25, 1989, U.S. Pat. No. 5,156,042 issued on Oct. 20, 1992, and U.S. Pat. No. 5,209,106 entitled "LEAK DETECTOR FLOAT SYSTEM AND METHOD THEREFOR", issued on May 11, 1993, set forth precision tank monitors which discloses problems involved in making highly accurate accounting of fluid products in storage tanks. These patents set out the problems involved in accurately detecting volume changes caused by environmental conditions such as temperature variation, changes in barometric pressure, and the like.
One of the most important steps in obtaining correct readings is positioning the float as close to its true buoyancy with respect to the product fluid before performing a level measurement of the float. The following two prior approaches involve mechanical solutions to positioning the float. U.S. Pat. No. 5,209,106 discloses a mechanical vibrator placed on the probe such that vibrational waves are delivered down the probe to break the surface tension existing between the float and the fluid. Once the surface tension is broken, the float positions itself within the fluid product based upon the buoyancy of the float in the fluid.
U.S. Pat. No. 5,220,310 issued to Pye on Jun. 15, 1993 pertains to a device having a motion-transmitting rod attached to a float. The rod extends upwardly through a steel core within a linear variable displacement transformer. A motor-driven cam applies a vibrational force that is transferred through a mechanical linkage including a pair of tension springs to the rod and float. After the vibrational force is removed, the float is subjected to three forces: the weight of the float, the float buoyancy force of the liquid, and the spring forces. The float responds to these forces by hunting up and down to seek an equilibrium. The range of the float travel is restricted to the practical range of linear variable displacement transformers and the practical length in which springs may be attached to the motion-transmitting rod member.
A need remains in the industry for a robust leak detector that has high accuracy and is insensitive to environmental conditions.
In general, the prior mechanical float excitation methods set forth above consume significant power due to the mechanical generation and coupling of the excitation signal. Mechanical excitation effectively breaks the surface tension of the float, but consumes significant power in the process. Because liquid level measurement tools are often powered by batteries and must operate reliably over a period of several days or weeks, power consumption is critical to long term usefulness and reliability. Also, the magnitude of voltage and current used to measure fluid levels in storage tanks containing volatile products such as gasoline, jet fuel, chemicals, etc. is of concern. A need exists for a leak detector with high efficiency and low voltage and operating current.
Mechanical excitation systems use motors, vibration transducers, and mechanical linkages that are subject to wear. Also, complex mechanical systems are difficult and expensive to manufacture and repair. Because leak detectors are moved often and must operate in harsh environments, it is desirable to minimize or eliminate any use of mechanical components to position the float.
Hence, a need exists to provide a system for achieving essentially true buoyancy of the float that (1) consumes little power and (2) does not use mechanical vibration while still providing a buoyancy system that operates in the harsh environment of a liquid storage tank. A further need exists to optionally provide self-calibration.
3. Solution to the Problem:
The problems set out above are solved by a leak detector that includes an energy efficient means for excitation of a float to repeatably position the float. The system of the present invention couples a high percentage of vibrational energy to the float itself so that power is not lost in mechanical linkages and vibration of other components. A leak detector that provides positive float movement and settling with respect to the surface of the fluid, that uses lower operating voltage and current and that increases efficiency and safety. The non-mechanical excitation system simplifies construction, transportation, installation, and maintenance of the leak detector. By minimizing mechanical linkages, errors caused by wear and thermal expansion of the components are limited. Also, by including a self-calibration apparatus, errors caused by temperature changes and the like can be accounted for making the leak detector highly accurate and robust.